Method of testing optical materials for microinhomogeneities

ABSTRACT

Solid optical materials such as glass which are to be used in laser systems may fail catastrophically due to the presence of microinhomogeneities. These microinhomogeneities are locatable by exposing the material to a laser pulse which preferably has a pulse duration of approximately 0.1 to 10 microseconds.

United States Patent Kantorslri et al.

[54] METHOD OF TESTING OPTICAL MATERIALS FOR MICROHNHOMOGENEITIES [72]Inventors: Joseph W. Kantorski, Southbridge, Mass;

Charles Gilbert Young, Storrs, Conn.

OTHER PUBLICATIONS Budin et aL: On The Dynamics of Laser-Induced Damagein IS 22 3O 24 2O 26 28 36 I4 38 [I6 wifjt i Eiliit 1 FebJLlWZ Glass,"Applied Physics Letters, vol. 9, No. 8, Oct. 15, 1966 PP. 29 l- 293Dupont et aL: On Laser-Induced Breakdown and Fracture in Glasses,"Applied Physics Letters, vol. II, No. 9, Nov 1 1967, pages 27] and 272Primary Examiner-Ronald L. Wibert Assistant Examiner-F. L. EvansAttorney-William C. Nealon, Noble S. Williams. Robert J. Bird andBernard L. Sweeney [57] 1 ABSTRACT Solid optical materials such as glasswhich are to be used in laser systems may fail catastrophically due tothe presence of microinhomogeneities. These microinhomogeneities arelocatable by exposing the material to a laser pulse which preferably hasa pulse duration of approximately 0.1 to 10 microseconds.

10 Claims, 8 Drawing Figures SAMPLE PATENTEUFEB 11912 3.639.066

' swan 1 or 2 FIG.|

INVENTOR. JOSEPH W. KANTORSKI CHQRLES GILBERT YOUNG PATENTEU FEB 1 I972SHEET 2 [IF 2 in km; QM.

m 01 #QE m at N 91 AGENT METHOD OF TESTING OPTICAL MATERIALS FORMICROINI-IOMOGENEITIES BACKGROUND OF THE INVENTION This invention isrelated to improved methods for testing solid optical materials and ismore particularly concerned with an improved method of testing suchmaterials for the presence ofmicroinhomogeneities.

A laser is a source of a highly coherent, high-intensity beam ofelectromagnetic radiation. The radiation is generally considered to belight although the output spectrum through which known laser materialsemit ranges from the infrared to the ultraviolet. Many types of lasersare capable, in certain configurations, of emitting a beam having a veryhigh-power density. These range from the very highaverage powercontinuous wave gas lasers to the Q-switched solid state laser. Theaverage power output is, of course, very different. However, the powerdensity during operation is the parameter of present interest.

Most solid optical materials, glass and crystal, have submicroscopicparticles which are not homogeneous with the surrounding mass. Many ofthese microinhomogeneities are of the order of l micrometer in diameteror less and as such are not discernible by visual means. When a laserbeam or other mode of high-energy density radiation is transmittedthrough the material, energy is absorbed by the material of themicroinhomogenity. However, the microinhomogeneities absorb energy at afaster rate than does the surrounding material. Therefore, the smalllocalized matter expands more quickly than does the matrix materialuntil a fracture occurs due to the thermal stresses which are set upvThis fracture may be very small or it may be catastrophic. If it issmall, it is still many times the size of the microinhomogeneity fromwhich it arose. When the material is again subjected to the beam, theenergy is now absorbed according to the area of the fracture. Since theabsorption is related to the area of the inhomogeneity, it can be seenthat under high-power density operation, eventual catastrophic failureis almost a certainty if any microinhomogeneities exist in the originalmaterial.

It is desirable, therefore, to be able to ascertain the condition of anoptical material for eventual use in such systems be fore the expensivefabrication process is commenced. Also, premature failures of consumeritems must be avoided in items of the complexity and cost of most lasersystems.

SUMMARY OF THE INVENTION It is, therefore, an object of the presentinvention to provide a method for testing solid optical materials forthe presence of microinhomogeneities.

Another object of the invention is to provide such a method which may beperformed on the solid optical materials prior to fabrication.

An additional object of the invention is to provide such a method whichwill allow the testing of a material without causing catastrophicfailures to occur.

A further object of the invention is to provide such a method in which apiece of material is exposed to laser radiation in a manner which doesnot cause additional damage to the volume of the material which isunassociated with the microinhomogeneities.

A still further object of the invention is to provide such a methodwhich may be performed quickly and inexpensively as a regular qualitycontrol operation.

Briefly, the invention in its broadest aspect comprises a method fortesting a piece of solid optical material for the presence ofmicroinhomogeneities. The method includes irradiating the piece ofsolidoptical material with a pulsed beam of coherent radiation having a pulsewidth in the range approximately between 50 nanoseconds and 500microseconds.

Further objects, advantages, and features of the invention will beapparent in the following specification taken together with theaccompanying drawing.

DESCRIPTION OF THE DRAWING In the drawing,

FIG. I is a schematic optical diagram of an amplified spon taneousemission laser which generates a laser pulse useful in the invention,

FIG. 2 is a plot of the signal input to the electronic Kerr cell devicein the laser apparatus shown in FIG. I, and

FIGS. 3 through 8 are plots of energy versus time at various locationswithin the laser apparatus of FIG. I.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As has been notedpreviously, a need exists for determining the presence ofmicroinhomogeneities in a piece of solid optical material. This isparticularly true of glasses which are doped with any of the variouselements which can emit laser radiation. Many of these glasses are usedto form rods for high-performance lasers and; therefore, they areextremely susceptible to damage as a result of any microinhomogeneitieswhich may be contained in the glass. At present, the majority of laserconfigurations in which the rods are operated near the damage thresholdare the Q-switched lasers. A Q-switched laser is one in which a resonantcavity is caused not to exist about a laser rod until a very high-stateof population inversion is present among the active ions. At this point,the cavity is made resonant and the energy stored in the excited ions isdumped" nearly instantaneously, generally under 50 nanoseconds.Therefore, the high-energy output and very short pulse duration providea high rate of energy deposition in the laser rod which can causefailure of the rod if the rod contains any of the microinhomogeneities.

Unfortunately, no practical means. of ascertaining the presence of thesemicroinhomogeneities by visual techniques has as yet been determined.The only way which is known at present is to expose the glass to laserradiation of an energy density which is high enough to cause small, butvisible fractures to occur at the damage sites. It has been generallysug gested that in order to attain this energy density that a Q-switched laser pulse be applied to the glass; however, it has been foundthat this is not generally satisfactory. The O- switched pulse can causedamage to the glass by two additional modes, i.e., surface damage andself-focusing damage.

Experimentally, in the Q-switched time domain, the threshold for damageto the output face of an untreated glass laser rod is always lower by afactor of two to five than is the threshold for internal damage. Neitherimmersing the rod end in water or optically contacting an undopedsection of glass to the rod end improves the situation. However,treating the glass either by a hydrofluoric acid etch or by washing itin dimethyldichlorosilane have produced a considerable increase in theresistance to damage; but the effect lasts for only a few minutes. Itcan, therefore, be seen that serious damage to the surface of the glassis possible if a Q-switched laser pulse is utilized to search forinternal microinhomogeneities.

Furthermore, it has been found that pulses of short duration, Q-switchedpulses, tend to be self-focused by a sufficient length of material. Thisself-focusing causes damage to the glass which is evidenced by a thin(about I micrometer diame ter) fossil track in the glass as well asdamage to the output face. This damage mechanism may be caused by anelectrostrictive effect caused by nonuniformity of the cross-sectionalintensity of the beam utilized for testing the glass.

To achieve a power density which is sufficiently high with a long pulselaser, having pulse duration of l millisecond or greater, requires atremendously larger and more expensive item of equipment such that thetest becomes unfeasible once again.

Additionally, it has been proposed to test a billet of laser glass byactive rather than passive means, e.g., cause the billet to lase. Thisis not practiced for several reasons. Among these is the fact that onlyone end of the billet, the output end,

would be fully tested due to the gain along the length of the billet.Therefore, the billet would become polarized, the output end of anyelements formed from the rod would have to conform to the output end ofthe billet, thus increasing the cost of the finished product.Furthermore, the billets generally are greater than 3 inches indiameter. Such a rod would be impossible to pump so that the center ofthe rod is as equally exposed to pumping energy as the outer layers.Therefore, only the outer portions would be useable as high-performancelaser rods.

However, it has been found that by irradiating the piece of materialwith a laser pulse having a pulse duration in the range approximatelybetween 50 nanoseconds and 500 microseconds, the aforementioneddifficulties may be overcome. In fact, the preferred embodiment of theinvention utilizes a pulse with a pulse duration approximately between0.1 and I microseconds. This pulse causes small discoid fractures at themicroinhomogeneities; however, due to the low number ofmicroinhomogeneities, it does not cause catastrophic failure of thoseglasses which are intended for eventual use in a high-intensity lasersystem.

An apparatus for producing a pulse of the proper duration for this testis shown schematically in FIG. 1 and is generally indicated by thereference numeral 10. This is an amplified spontaneous emission laserand is comprised ofthree sections, a generator 12, a driver 14, and apower amplifier 16. The generator 12 is, in turn, comprised ofa pair ofserially aligned neodymium doped glass laser rods 18 and 20. Each of therods is a 1 meter in length by 15 millimeters in diameter glass laserrod which has 4 weight percent of neodymium doping within the 10millimeter diameter core. The cladding of the Brewster-ended rodsabsorbs off-axis emission, transmits the pumping wavelengths, isthermally matched to the core, and has an index of refraction, at 1.06micrometers, which is one part in 700 higher than that of the core.Total internal reflection of off-axis amplified spontaneous emission is,therefore, prevented and there is a negligible Fresnel reflection at theinterface.

The neodymium doped laser rods 18 and 20 are in series optically withfour Faraday rotation isolators 22, 24, 26, and 28 and two Kerr cells 30and 32. The Faraday rotation isolators are described in copendingapplication, Ser. No. 838,678, by C. Gilbert Young, a coinventor herein,and is assigned to the same assignee as is the instant application. Thefirst two isolators 22 and 24 are sandwiched about the first Kerr cell30 and are located optically between the rods 18 and 20. In like manner,the other two isolators 26 and 28 are sandwiched about the second Kerrcell 32 and are located optically after the laser rod 20. The opticalisolators prevent feedback and thereby lend gain stability to thesystem. The Kerr cells serve to truncate and shape a portion of theamplified spontaneous pulse in the desired manner. The Kerr cells aredriven by an associated electronic circuit 34 which supplies the signalshown in FIG. 2 to the Kerr cells 30 and 32. The amplified spontaneouspulse which is emitted from the first laser rod 18 is shown in the plotof FIG. 3. The input pulse to and the output pulse from the second laserrod 20 are shown in FIGS. 4 and respectively. As can be seen in FIG. 6,the output pulse from the generator 12 has the desired pulse shape. Itshould be noted at this time that by changing the particular Kerr cellsand by adjusting circuit 34 that the duration of the pulse may be freelyaltered to meet test requirements. However, it should also be noted thata shortening of the pulse duration inherently lowers the energy outputfrom the apparatus.

Following the generator 12 is a first afocal telescope 36 which reducesthe beam spread and matches the beam diameter to the l8-millimeterdiameter of the l-meter long driver rod 14. The driver rod 14 is dopedwith approximately 3 weight percent of neodymium. The amplified outputpulse from the driver rod 14 is shown in FIG. 7.

The driver output pulse passes through a second afocal telescope 38having the same basic functions as the first afocal telescope 36. Thebroadened beam now enters the power amplifier rod 16. The plot of theamplified output pulse therefrom is shown in FIG. 8. This pulse is thencollected by a lens 40 and the sample 42 is exposed to the resultingbeam.

As can be seen from FIG. 8, this apparatus can provide the desired pulsefor this test. The pulse has approximately joules of energy and lastsapproximately 10 microseconds. This has been found to be a satisfactoryenergy content for this duration of pulse; however, at the shorter pulsedurations, lower energy levels are necessary. The effective range ofenergy levels generally is between 10 and 200 joules. The test pulseshown in FIG. 8 is free of any spikes and is quite uniform across thebeam.

This type of pulse is very advantageous for this test for severalreasons. Among these is the elimination of self-focus ing damage becauseofthe beam uniformity; surface damage is eliminated due to the farhigher damage threshold for longer pulse lengths; the threshold forinternal damage due to inclusion of microinhomogeneities is only raisedby a factor of about two; the size of the failures may be controlled byvarying the pulse duration; and the entire piece of material may betested uniformly without destroying the billet.

The normal as-cast billet of laser glass is approximately 40 cm. incross-sectional area; therefore, the entire area of the billet cannot betested simultaneously with the above apparatus. The crosssectional areairradiated per scan with the above apparatus is approximately 0.5 cm.and the entire cross-sectional area is scanned by repetitive testing.However, depending upon the power of the apparatus, it is possible totest an entire billet at once.

Applicants have found that the lO-microsecond pulse duration causes amaximum size fracture for a given small microinhomogeneity size. Forexample, for a certain size of inclusion, the lO-microsecond pulsecauses a fracture ofa few centimeters in diameter, a l-microsecond pulseyields a fracture diameter of a few millimeters, and a 0.1-microsecondpulse causes fractures having only a few tenths of a millimeterdiameter. It is generally attempted to prevent the discoid fracturesfrom becoming too large, the purpose of the test being only to locatethe included microinhomogeneities. However, the fracture size must belarge enough so that it can be seen within the material. In a billet oflaser glass where erbium is the active dopant, the fracture size must besomewhat larger due to the poor visual transmittance of this glass.Therefore, the test may be adjusted to match the probable condition ofthe rod.

A technique which is often utilized is to irradiate the entire billetarea two or more times. This procedure serves to locate even many verysmall inclusions because the first scan causes a small localized growthof the inclusion and the second scan causes a fracture large enough tobe seen by an observer. This double-exposure process greatly increasesthe changes of locating even very minute inclusions.

Once a billet has been exposed to the radiation, it is visuallyinspected for the characteristic discoid fractures associated with themicroinhomogeneities. The failure is generally not catastrophic underthe above energy densities and pulse durations and the billet may oftenbe salvaged by forming the finished optical elements from the undamagedportions of the billet if the fracture size is kept to a minimum. Thisgreatly reduces the cost of the resulting elements because failures atlater stages of testing due to the microinhomogeneities are eliminated.

Although the above description has been limited generally to glasses andparticularly to laser glasses, it is quite obvious that this method isequally applicable to all solid optical materials which are transparentto the radiation utilized. The source of this radiation need not bematched to the material to be tested in any other manner excepttransparency.

While there have been described what are considered to be preferredembodiments of the present invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention as defined in the appendedclaims.

Iclaim:

l. A method of testing a piece of solid optical material for thepresence of microinhomogeneities, which comprises irradiating the pieceof solid optical material with a pulsed beam of coherent radiation, eachpulse having an approximately square shape and having a pulse width inthe range approximately between 50 nanoseconds and 500 microseconds andinspecting the piece of solid optical material for discoid fracturesfollowing irradiation by the pulsed beam of coherent radiation.

2. A method according to claim 1, comprising sequentially irradiatingportions of the piece until all portions have been exposed to theradiation.

3. A method according to claim 1, comprising irradiating said piece aplurality of times by the pulsed beam of coherent radiation.

4. A method according to claim 1, comprising irradiating said piece twotimes by the pulsed beam of coherent radiation.

70. A method according to claim 1 comprising restricting the pulse widthto the range approximately between 0.1 microsecond and I0 microseconds.

6. A method according to claim 5, comprising restricting the energycontained in the pulse to the range approximately between 10 and 200joules.

7. A method according to claim I, comprising irradiating a glass.

8. A method according to claim 7, comprising irradiating a doped laserglass.

9. A method according to claim 1, comprising irradiating a crystallinematerial.

10. A method according to claim 9, comprising irradiating a dopedcrystalline laser material.

1. A method of testing a piece of solid optical material for thepresence of microinhomogeneities, which comprises irradiating the pieceof solid optical material with a pulsed beam of coherent radiation, eachpulse having an approximately square shape and having a pulse width inthe range approximately between 50 nanoseconds and 500 microseconds andinspecting the piece of solid optical material for discoid fracturesfollowing irradiation by the pulsed beam of coherent radiation.
 2. Amethod according to claim 1, comprising sequentially irradiatingportions of the piece until all portions have been exposed to theradiation.
 3. A method according to claim 1, comprising irradiating saidpiece a plurality of times by the pulsed beam of coherent radiation. 4.A method according to claim 1, comprising irradiating said piece twotimes by the pulsed beam of coherent radiation. %. A method according toclaim 1, comprising restricting the pulse width to the rangeapproximately between 0.1 microsecond and 10 microseconds.
 6. A methodaccording to claim 5, comprising restricting the energy contained in thepulse to the range approximately between 10 and 200 joules.
 7. A methodaccording to claim 1, comprising irradiating a glass.
 8. A methodaccording to claim 7, comprising irradiating a doped laser glass.
 9. Amethod according to claim 1, comprising irradiating a crystallinematerial.
 10. A method according to claim 9, comprising irradiating adoped crystalline laser material.